U.S. Army surveillance aircraft are joining the Washington area hunt for a deadly sniper who has eluded law enforcement authorities for two weeks.

Pentagon officials have been hesitant to give more than the most basic of details about the military's involvement in the sniper hunt since Defense Secretary Donald Rumsfeld formally approved the action late Tuesday.

But defense sources have disclosed to reporters the main military contribution will be Army surveillance planes.

One is understood to be the four-engine turboprop RC-7 "Airborne Reconnaissance Low" aircraft.

According to the Army's official weapons system guide, that plane is capable of detecting, locating and reporting ground activities by people and vehicles using a variety of sophisticated sensors. It was designed in part for counter-narcotics missions in Central and South America.

The RC-7 is a modified DeHavilland DH-7, with day and night, all-weather flying capability. Experts say it can remain in the air for 8-10 hours at a time.

Pentagon officials say the missions will begin as soon as possible. But they are declining to say precisely when so as not to alert the sniper.

The plan calls for military pilots to fly the Washington area reconnaissance missions accompanied by federal agents, who would relay to police on the ground information, for example, on the movement of a suspicious vehicle or even the detection of gun muzzle flashes.

Defense officials stress their help was requested by the FBI, the Federal Bureau of Investigation. They also stress the military is only acting in a supporting role and is thus in compliance with the 19th century law that restricts the military's involvement in domestic law enforcement.